Native osmium

--- from the Neiva River, Russia

Figures 1-2.
Two multi-layered composite images of native osmium grains,
taken in reflected light with a stereomicroscope, nominal magnification 25X,
long-axis field of view (somewhat trimmed here) 5 mm.
The two images show two grains, nicknamed "the tile" and "the asteroid". Both sides of each grain
are shown: respective sizes are approximately
2.6x1.7x0.8 mm and 3.0x1.8x1.2 mm. The combined weight of
the two grains is less than 0.01 grams.
Specimens from
Gunnar Färber Mineralien,
Germany, at the Denver show, September 2015.

"Rock of the Month #177, posted for March 2016" ---

Native Osmium

from the Urals of Russia. Precious metals often occur as small grains and flakes in
river gravels, so-called placer deposits. The placers are concentrates of heavy mineral
grains, and osmium is the densest of all the elements and a
platinum group mineral (PGM).
The location is given as "Neiva River, Newjansk, Ekaterinburg"
(Nizhnii Tagil, Sverdlovskaya oblast, Middle Urals).
The Ural Mountains, parts of Siberia and Kamchatka are all noted placer PGE
(platinum group element) districts.

The Mineral

The silvery flakes do not show a clear crystal habit, though
the thin "tile" appears to preserve parts of four crystal faces.
This is a good clue:
osmium is hexagonal,
and some crystals exhibit perfect hexagonal outlines.
Osmium forms alloys with other PGE,
especially iridium and ruthenium, as in the species iridosmine.
The "asteroid" grain responds strongly to a powerful magnet,
whereas the "tile" flake does not.

An electron microprobe or microscope
examination is in order, to prove the identity as osmium, and to see what
other elements or mineral species may be present. Iron may alloy with PGE,
most particularly in the case of platinum itself: could this explain the
difference in magnetic behaviour (?). The "tile" may be
a corroded osmium crystal, whereas the "asteroid" may be a combination
of associated metallic mineral species.

Native osmium (Cabri, 2002, pp.76-77) commonly forms
thin hexagonal platelets, often exsolved within grains
of Pt-Fe alloys, such as isoferroplatinum,
Pt3Fe.
There is complete solid solution with ruthenium, and partial
solid solution towards iridium.
Some grains now defined as osmium are referred to as iridosmine
in older publications.
Weiser (2002) provides SEM images of fine examples of osmium
crystals and platelets from Colombia and Burma.

A note on the Ural Placers

Placers of the
Nizhnii Tagil dunite
massif, located circa 115 km N.N.W. of Sverdlovsk,
display varied mineralogy
(Cabri and Genkin, 1991).
The placer grains include isoferroplatinum with laths of osmium
and grains of iridium, chromite, as well as laurite and other PGM species.
The Ural Mountains are the surface expression
of an eroded orogenic belt, with at least 15 major
ultramafic complexes extending for
about 900 km northwards from Ekaterinburg,
at 60°E, 56-64°N (see map in Garuti et al., 2002a,b).
These include tulameenite, a secondary phase
formed during serpentinization of ultramafic massifs. With an ideal
formula of
Pt2FeCu,
it is usually found as a rim on primary Pt alloys.
Associated minerals include
laurite, tetraferroplatinum, isoferroplatinum and native osmium.
Some of the PGM, including Pt-Fe alloys,
found in the Urals placers may be derived from chromitites in the
ultramafic complexes (Garuti et al., 2002a,b: see also
Okrugin, 2011).

Worldwide Occurrence and Distribution

Native osmium and associated PGM are commonly derived from
zoned, "Alaskan-type" mafic-ultramafic intrusions, or with
sub-seafloor ophiolite complexes.
Weiser (2002) reviews the kinds of PGM species found as
heavy grains in placer deposits worldwide.
Localities are documented worldwide, in such places as:
the Choco district of Colombia;
British Columbia, Yukon and Alberta, Canada;
Alaska and California, U.S.A.;
Ethiopia;
Sierra Leone;
Burma;
South Kalimantan, Borneo, Indonesia;
Papua New Guinea;
Tasmania, Australia;
and a number of localities across the vastness of Russia.

Osmium-bearing grains were also recovered during the California
gold rush. White grains of heavy mineral collected from gold
samples in 1849-1850 were found to be largely Pt, Ir and Os.
Some of this was native Pt, while some was at the time referred
to as sisserskite or sysertskite, varieties of iridosmine and native
Os (Genth, 1853). These included "brilliant lead-coloured scales,
some of which were imperfect six-sided prisms", highly suggestive
of osmium.

According to the informative summary by O'Neill and Gunning (1934, pp.115-122)
placer platinum was discovered in the Urals in 1823, and production began the
following year. These authors reported that nine ultramafic centres,
pyroxenite and especially dunite, were of economic importance,
giving rise to platiniferous placers. The olivine of the host rock weathers
readily, and is seldom found in the placer grains, though chromite may be attached to the PGM.
Native platinum associated with pyroxenite may be especially irregular in habit,
and may have attached magnetite.
The first Urals discovery may actually date to 1819 (Okrugin, 2011, p.1409),
and, as mentioned above, there are at least 15 significant ultramafic complexes
in the Middle and Northern Urals.
In the early days of the Russian placer mining operations, a number of
large nuggets (>1 kg) were recovered (e.g., Okrugin, 2011).

One of the other Russian occurrences may be noted here.
The Guli
placer deposit contains native osmium and iridosmine. The Guli
intrusion, in the northern Siberian craton,
may be the largest clinopyroxenite-dunite complex on Earth.
It has an exposed area of about
600 km2,
including 380 km2
of dunites. This may be the world's largest exposure of dunites: the
total of all dunites in the Ural PGE belt is estimated at 100
km2
(Lazarenkov et al., 1994).

Native osmium is comparatively rare: as of 01 November 2015,
the MINLIB bibliographic database
held 5,964 records on all PGE, of which 1,343 referred to
osmium, including 510 to osmium isotopes (used in the rhenium-osmium
geochronometer) and just 44 for native osmium, though a few more
would doubtless be found under the older names of iridosmine / osmiridium.
Osmium is composed of seven stable (or exceedingly long-lived) isotopes,
184, 186, 187, 188, 189, 190, 192Os.

Osmium the Element: the Metal and its Uses

Osmium was discovered by Smithson Tennant in 1803 (Newton Friend, 1961).
The silvery metal may be volatilised as the tetroxide,
OsO4,
a fact which must be considered by analysts to this day.
Explorers for PGE and related base metals commonly assay for
platinum and palladium alone, as a cost-saving matter,
reasoning that the other four, less-common PGE are unlikely
to be present in significant quantities if Pt and Pd
are not abundant.
Commercial assay labs often add
gold to offer a convenient 3-element, precious-metals "package".

Osmium is element 76, atomic mass 190.2,
Pure osmium has a specific gravity of 22.59.
This makes it the densest element known, though iridium is
not far behind (Street and Alexander, 1998, p.208).
Recalculated specific gravities of Os and Ir are 22.590 and 22.560
respectively, which confirmed Os as the densest metal (Arblaster, 1989).
Osmium also is one of the
most refractory elements, with a melting point of 2700°C.
Despite this, it oxidizes readily, and thus may weather
more readily than other PGE.

Osmium has been recovered in placer deposits in the Urals and elsewhere,
as a heavy mineral in the Witwatersrand gold fields
of South Africa, and in concentrates from a number of
nickel-copper-PGE mines.
The annual world production of osmium is so small, perhaps
on the order of one tonne (about 30,000 ounces),
that
uses are necessarily limited (Pt and Pd, the most abundant PGE, are
used extensively as catalysts in automotive exhaust systems
and the chemical industry, as well as in jewellery and dental alloys).
The metals business reports in detail on Pt and Pd, and sometimes Rh,
rather less on Ir and especially Ru and Os.

Osmium has been used in speciality alloys of extreme hardness,
in "iridium" tips
for fine fountain pens, as an electroplated reflective coating for
searchlights, and in light filaments (Newton Friend, 1961).
Os-It-Pt alloys find use in some electrical contacts.
Hard alloys may also be used in gramophone needles and
in fine instrument pivots. Osmic acid
is used as a tissue stain for electron microscopy
(Simons, 1967, pp.128-130; Gray, 2009, pp.174-175).